FILE No.71Unsolved
A hunter must take a dog, a rabbit and a carrot across a river. His boat holds himself plus just one item.
On a bank without the hunter, the dog and rabbit alone means the dog attacks the rabbit, and the rabbit and carrot alone means the rabbit eats the carrot. The dog and carrot alone are fine.
Each trip across the river counts as 1 crossing (in either direction).
What is the fewest number of crossings needed to get everything across safely?
If you get stuck, open the hints one at a time, in order.
Which item can be left behind first without trouble?
After taking the dog over, can you leave the rabbit with it?
Bringing something back is allowed.
What is the fewest number of crossings needed to get everything across safely?
Choose the answer closest to the truth.
7
The answer is: 7 crossings.
1) Take the rabbit across. 2) Return alone. 3) Take the dog across. 4) Bring the rabbit back. 5) Take the carrot across. 6) Return alone. 7) Take the rabbit across.
Taking the rabbit first is the key; the dog and the carrot are safe together. The rabbit is brought back on crossing 4 so that it is never left alone with the dog or the carrot.
People assume cargo never comes back, so the "bring it back" move does not occur to them.
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